Casual observation shows roadway construction to be an on-going exercise in most communities, especially in wet climate areas and those susceptible to repeated freeze-thaw cycles. Engineering and performance failures of pavement compositions eventually require corrective action. Remedial measures to repair cracks, potholes and the like offer only a short-term solution to the problem of distressed roadways. Ultimately, complete restoration is necessary, but only at great cost and inconvenience, in terms of time, disrupted traffic patterns, and over-extended tax revenues.
A concurrent problem relates to the accumulation of asphalt roofing wastes. Each year, according to a recent estimate, approximately 92 million squares of roofing shingles are produced in the United States. A square, by definition, is material sufficient to cover 100 square feet of surface area including overlap (typically, 240 sq. ft. without overlap), typically contains 80 shingles, and depending upon the particular shingle composition weighs between 200-250 pounds. Shingles produced after 1980 are 20-30% asphalt by weight, with the remainder comprising fiberglass mat, roofing granules, filler, and back-surfacing materials. Those produced prior to 1980 contain roughly 1.5 times the amount of asphalt.
Significant waste accompanies the manufacture of such materials. Broken and defective shingles are discarded. Production of standard three-tab shingles produces cut-outs and trimmings which total nearly 1% of their total weight. Based on annual shingle production figures (and using post-1980 composition data), approximately 100,000 tons of shingle cut-outs/trimmings and 25,000 tons of asphalt are generated each year. The total amount of broken and defective shingles discarded annually is of a comparable magnitude.
An additional facet of the situation is that a significant amount of previously-applied shingles are recovered each year. The typical residential roof may be re-shingled two or more times before all old shingles are removed. On an annual basis, approximately 2.1 million tons of asphalt are generated each year in this manner--in addition to the manufacturing wastes referenced above.
The annual amount of waste attributable to shingles provides only a glimpse as to the real scope of the problem. Other types of asphalt roofing materials include asphalt-saturated organic felts, asphalt-impregnated glass and polyester mats, rolled roofing products such as ply sheets, modified bitumen membranes and the like, as well as commercial build-up roofs (alternating layers of asphalt and prepared rolled roofing products on commercial buildings). The manufacture and replacement of each has associated with it a significant amount of waste. (For the purpose of this discussion, the term "asphalt roofing wastes" will be used to refer to waste generated through the manufacture and/or disposal of these and other materials.)
For many years, asphalt roofing wastes have been land-filled. What once seemed to be a safe and sound "solution" spawned a variety of more worrisome concerns, foremost among which is the creation of permanent landfills--land which over time becomes non-reclaimable. Furthermore, with an increasingly-larger population and ever-growing volume of solid wastes, the number of available landfills has dwindled to the point where most states now ban certain types of refuse. For these reasons, it is no longer environmentally-wise or economically-feasible to continue landfill disposal of asphalt roofing wastes.
The search for a beneficial alternative to land-fill disposal of asphalt roofing wastes has been on-going concern in the art. One such use is in the field of roadway construction. By way of background, pavements are typically only about 4-6% asphalt binder by weight, the remainder comprised primarily of aggregate. Conventional approaches, which have been used with very limited success, merely attempt to offset the amount and cost of asphalt used with recycled roofing waste.
In U.S. Pat. No. 4,325,641, a method of processing asphalt shingles is described, whereby a hot mix pavement is produced in which about 50% by weight of the asphalt component may be substituted with asphalt shingles. (For the purpose of this discussion, a "hot-mix" asphalt pavement is comprised of aggregate mixed and coated with an asphalt binder; heat applied prior to mixing dries the aggregate and gives the asphalt sufficient fluidity.) A similar approach, discussed in U.S. Pat. No. 4,706,893, also relates to a method of processing asphalt shingles such that only about 40% of the asphalt component--or about 2% of the total weight of the composition--may be derived from shingle material.
However, the prior art has associated with it a number of obvious and significant problems and deficiencies. Most are related to pavement performance and result from the roofing wastes used therewith. Asphalt pavements of the prior art may be characterized by a number of predictable performance deficiencies, which, if left uncorrected, may eventually lead to complete roadway failure.
A major problem is that methods and compositions of the prior art use roofing waste merely as an asphalt substitute. While the environmental and economic benefits derived from diverting a hazardous waste stream are obvious, use of it in this manner has not contributed anything to pavement performance or the engineering properties of the asphalt component.
A related problem of the prior art stems from the fact that a pavement composition must be designed in such a manner that pertinent physical properties may be measured with some degree of precision and correlation to actual roadway performance. Many government units, including state highway agencies, have adopted such an engineering-based approach. (Several are used, but the most widely-accepted approach is known in the art as the "Marshall Method".) As a result, a pavement composition based solely on a "recipe" formulation which does not correlate well to eventual performance is undesirable. It is an advantage in the context of a competitive bidding process to be able to provide comparative data to support improved performance properties.
The problem arises in that pavement compositions of the prior art which incorporate roofing waste material do not yield test data which, in any way, indicate improved performance--whether or not compared to pavements without such materials. The lack of such favorable data is reflected in the marginal performance eventually observed.
Another major problem relates to the fact that an asphalt pavement is predominantly asphalt-coated aggregate. The aggregate is of a size, cut, and porosity which imparts to it a relatively large surface area per unit volume. In order to ensure adequate coating, a high-penetrating asphalt is needed, one which is readily fluid at process temperatures of 300.degree.-400.degree. F. The temperature-stable, oxidized asphalts typically found in roofing products are not readily fluid within this temperature range and present problems and subsequent mix design difficulties if substituted for conventional paving grade asphalt. As a result, lower-viscosity paving grade asphalts and/or recycling oils must be used to offset the increased viscosity caused by any roofing waste present.
With low asphalt viscosity, the aggregate surface is coated with only a thin film of asphalt which is quite susceptible to oxidative aging. A minimal coating also increases the degree of void space between individual aggregate particles. High void content, in turn, increases the permeability of the composition to air and water, further causing premature asphalt hardening.
Another deficiency of the prior art is "flushing"--the phenomenon of asphalt actually oozing out of a pavement composition. The low viscosity and high-penetration qualities of an asphalt which promote aggregate coating will cause a pavement to become slick and oily at normal summer temperatures. Conventional wisdom indicates that limiting the amount of roofing waste in a pavement composition will lower viscosity and improve processing. However, such limitations do nothing to alleviate flushing and related stability concerns.
Performance deficiencies of the prior art are, in part, attributable to the fact that the roofing shingles used are about 1/8 inch or larger in cross-section. At this size the roofing waste does not become fully integrated into the asphalt component. Rather, it behaves as a sticky aggregate, causing additional mix design difficulties.
In summary, there are a considerable number of drawbacks and problems relating to the use of recycled asphalt roofing waste in a roadway pavement composition. There is a need for a composition which not only provides an economic alternative to landfill disposal, but also a pavement with enhanced performance properties.